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sizes for exhaust and air return

Hi all,

I need to get a replacement a/c and furnace system for my 2 story 2000 sq ft home built in 1980. I had two local contractors (both well recommended) come out and got somewhat different inputs but both suggest a 4 ton system which equals the original capacity. Main items I have questions on:

1) flow area for return air: Original equip had a 30 x 10 inch register. We placed a built in cabinet (it's a long story) that has a 3.5 x 42 inch area for return air. It is positioned about 3 inches in front and 5 inches below that 30 x 10 register. Our existing system did work with this, but first contractor thinks it's probably OK, the second says it needs to be opened up. It would require $500 of rework to that cabinet to install some kind of decorative grill in place of the solid facade panel currently there. I was thinking that I could take a wait and see on this one as it can be modified after the new unit is installed. Is there a definite way to determine what's adequate?

2) The exhaust stack size. Currently have original thing in place which is a heavy wall "pipe" 4.2 inch inside diameter, I'm told it likely contains asbestos. Goes from the ceiling in the garage straight up thru the roof, about 20 ft long. Current arrangement has both the furnace and 50 gallon gas water heater exhausts Y connected just below and sharing that passage. First contractor doesn't bring it up, second guy says it's not adequate for both. I think he said that current code requires a 4" metal vent for the furnace alone. Any way to know if my built in "pipe" will be adequate for both the furnace and water heater?

The 4" flue is only good for the water heater. I assume the 50 gal. W/H vent is 4"? You could go with a "90 plus" efficiency (side vent) furnace or install a larger flue if you stay with a 80% furnace.

Regarding the A/C......4 tons sound way too big for a 2,000 sq. ft., 2-story home. How many returns are there in your home? Do you have a basement or a crawl space?

I need to get a replacement a/c and furnace system for my 2 story 2000 sq ft home built in 1980. I had two local contractors (both well recommended) come out and got somewhat different inputs but both suggest a 4 ton system which equals the original capacity. Main items I have questions on:

1) flow area for return air: Original equip had a 30 x 10 inch register. We placed a built in cabinet (it's a long story) that has a 3.5 x 42 inch area for return air. It is positioned about 3 inches in front and 5 inches below that 30 x 10 register. Our existing system did work with this, but first contractor thinks it's probably OK, the second says it needs to be opened up. It would require $500 of rework to that cabinet to install some kind of decorative grill in place of the solid facade panel currently there. I was thinking that I could take a wait and see on this one as it can be modified after the new unit is installed. Is there a definite way to determine what's adequate?

2) The exhaust stack size. Currently have original thing in place which is a heavy wall "pipe" 4.2 inch inside diameter, I'm told it likely contains asbestos. Goes from the ceiling in the garage straight up thru the roof, about 20 ft long. Current arrangement has both the furnace and 50 gallon gas water heater exhausts Y connected just below and sharing that passage. First contractor doesn't bring it up, second guy says it's not adequate for both. I think he said that current code requires a 4" metal vent for the furnace alone. Any way to know if my built in "pipe" will be adequate for both the furnace and water heater?

Thanks for your assistance.
Conrad

if thats the only return you got its not big enough. do you know what size the duct is behind the return? or returns? if you got other returns what size are they?

My current water heater vent looks like a 2 or 2.5 inch dia. My neighborhood is tract housing and just by walking thru I can tell that every home identical to mine still has that same 4.25 ID pipe punching thru the roofs and no separate vent for water heaters. Cost to remove the asbestos pipe and replace with a larger vent is substantial so I'm looking for some solid "proof" to convince me to pay for that. Is the required size a specific calculation or more of a judgement call? Obviously most of my neighbors are sticking with the original 4.25 "pipe" and I know most have replaced their furnaces and most use nat gas water heaters so I find this confusing.

Originally Posted by jacob-k

if thats the only return you got its not big enough. do you know what size the duct is behind the return? or returns? if you got other returns what size are they?

The return register 30 x 10 inches is the only opening to a boxed in volume approx 3' x 4' x 2' high directly below the furnace / blower. That "box" is just the floor of my garage with the furnace sitting on the top deck 2' above it.

Here's one of the confusing points for me: These are all tract homes, built in 1980, which had to comply with existing codes at that time and pass inspections. I have not added to the size of the house, still using same nat gas heating and electric A/C, still had same 4 ton capacity proposed as replacements, so why would required sizes of exhaust vents and return registers change?

A 3" vent on the 50 gallon water heater? The 4.25 inches you're stating for the flue must be a 5" (I've never seen odd numbers on a flue) and at 20+ feet it'll carry 155,000 btus. What size is the furnace?

Jacob-k is correct...........the 30"x10" return is not large enough. I don't care that every home is the same or that it has "worked" all these years. It is killing the efficiency and output of the equipment. Also, I still can not believe that a 4 ton is what the home needs. Unless it is extremely hot in Irvin, CA. and the homes have no insulation, poor windows and no attic ventilation. I A/C a 2,250 sq. ft 2-story with a 2.5 ton as an example. I'm sure our humidity is higher.

Also, know this, most "salesmen" will size the equipment according to what the home has in it now. This is completely irresponsible! If the power companies were truly concerned about peak efficiency for the homeowners (via rebate incentives), they would require that a Manual J and Manual D be completed on every home before rebate dollars were issued.

Is there a forced power vent on the water heater and the furnace? Power venting increases the capacity allowed through the vent. Also the vent material makes a difference. Since it was used in multiple houses in a tract setting, chances are it was an engineered design calculated to show the code official that it would work. The code allows that option. Apparently it does work for the system that was installed. 33 years is decent life expectancy for that type equipment. Same goes for the return air. As long as the total external static pressure at design CFM of the fan is not exceeded, the return opening can be small. And again, that is probably the case because of the tract situation. The design was probably engineered, calculated and shown to work so it could be used multiple times to save first cost money, with little thought to energy savings. Not to say that larger or more openings wouldn't work better especially in the case of the RA grille.

Here's one of the confusing points for me: These are all tract homes, built in 1980, which had to comply with existing codes at that time and pass inspections. I have not added to the size of the house, still using same nat gas heating and electric A/C, still had same 4 ton capacity proposed as replacements, so why would required sizes of exhaust vents and return registers change?

Building codes typically are concerned with safety. Examples would be how and where you may run gas and electrical lines. Clearances to combustibles etc. They have very little to do with equipment sizing. This seems to be changing in some jurisdictions, but the process is slow.

Undersized ductwork is a common problem as is oversized equipment. These aren't really covered by building codes.

Is there a forced power vent on the water heater and the furnace? Power venting increases the capacity allowed through the vent. Also the vent material makes a difference. Since it was used in multiple houses in a tract setting, chances are it was an engineered design calculated to show the code official that it would work. The code allows that option. Apparently it does work for the system that was installed. 33 years is decent life expectancy for that type equipment. Same goes for the return air. As long as the total external static pressure at design CFM of the fan is not exceeded, the return opening can be small. And again, that is probably the case because of the tract situation. The design was probably engineered, calculated and shown to work so it could be used multiple times to save first cost money, with little thought to energy savings. Not to say that larger or more openings wouldn't work better especially in the case of the RA grille.

I would be very surprised and extremely impressed with the engineer that made a 10x30 return on a 4 ton system operate at .5 TESP...

Building codes typically are concerned with safety. Examples would be how and where you may run gas and electrical lines. Clearances to combustibles etc. They have very little to do with equipment sizing. This seems to be changing in some jurisdictions, but the process is slow.

Undersized ductwork is a common problem as is oversized equipment. These aren't really covered by building codes.

Great point on codes. Sometimes I scratch my head with them though. Example gas furance in a closest in my area since combustible most be seting on 5/8 Sheetrock/firerock. Ok I understand that but what about the wooden door in front of it that opens and shuts? It wood so if the worst case thing happened and the furance had roll out the door is right there and would catch fire first.

Just saying that some codes don't make sense and others do. My example is just something I see all the time and as long as we replace the platform or add 5/8 Sheetrock, new gas Cock and gas flex we meet code.

I wish code for installing a new system in existing home or new home would require manual j and manual d load cals. While this has nothing to do with safety other then oversizing a gas furance that might expersene a cracked heat exchanger because the duct work is to small.

Code just applys to safety in my area nothing else. So anyone that meets the basic code will be pasted but noone every looks at the size if the equipment, return, supply etc...

I do know that in some states they require manual j and d cals for rebate programs but not in my area. So often 500 square foot rule gets applied by hvac contractor along with other rules of thumb to size ductwork. Untill every state requires this on every job things will continue on as they have for years. The customer is the one that gets stuck holding the bag when it's sized wrong.